This invention generally relates to sootblowers which are used to project a stream of a sootblowing medium against internal surfaces of a combustion device. In particular, this invention concerns a hub assembly which provides sealing between a stationary sootblowing medium feed tube and a relatively moveable lance tube.
Sootblowers are used to project a stream of cleaning medium such as water, air or steam against heat transfer surfaces within a combustion device such as large scale boilers to cause slag and ash encrustations to be removed. The cleaning medium impact produces mechanical and thermal shock which causes these adhering layers to be removed. One general category of sootblowers is known as the long retracting type. These devices have a retractable lance tube which is periodically advanced into and withdrawn from the boiler, and is often simultaneously rotated such that one or more cleaning medium nozzles on the lance tube project a jet of cleaning medium tracing a helical path. In typical sootblowers, a feed tube is held stationary relative to the sootblower structure. One end of the feed tube is supplied with the cleaning medium through a poppet valve. The sootblower lance tube slidably over-fits the feed tube and its longitudinal sliding and rotational motion is controlled by a carriage. The carriage moves along a toothed rack to move the lance tube longitudinally. The sootblowing medium supplied to the feed tube in turn pressurizes the inside of the lance tube with the sootblowing medium.
To prevent the escape of sootblowing medium from any area other than the nozzles which are oriented to project the sootblowing stream in a desired manner, a packing is provided in a stuffing box between the feed tube and lance tube. This packing is typically incorporated into a hub within the sootblower carriage which is used to drive the lance tube mechanism. Various types of packing material are presently employed. In todays practice, graphite foil type packing materials are frequently used. In order to generate the desired sealing action between the hub and feed tube, it is necessary to apply an axial force on the packing material. This force is normally provided through the use of a packing gland having clamping bolts which transfer a clamping force against the packing, causing the packing to be squeezed into engagement with the feed tube and hub. As the packing material wears, the degree of initial axial force or preloading which is provided by the packing gland is often lost. This force loss can result in leakage through the packing, which is undesirable.
In order to allow a certain degree of packing wear without leakage, it is ordinary procedure that axial loads are placed on the packing at the time of adjustment which exceeds that necessary to provide proper sealing. Such excessive loads allow a degree of packing wear without causing leakage. Such excessive axial loads result in higher wearing of the packing and produces packing friction against the feed tube which exceeds that which is necessary for sealing, resulting in increased power requirements for sootblower actuation. The need to provide a desired preload on the packing is also a maintenance concern since, for many sootblowers, it is necessary to periodically, and even daily, tighten the packing gland to keep the packing from leaking.
One approach toward gaining increased life of packing without the frequent maintenance of manually setting the packing preload, is to use a compliant element such as a spring for actuation of loading of the packing. Ideally, the compliant element would be capable of a considerable degree of displacement due to packing wear while providing an actuating force transferred to the packing within a desired range. Various types of springs could potentially be used, for example, a stack of Belleville washers, coil springs or wave type springs, etc. The desired force versus displacement relationship of such springs dictates a particular free spring length. If it is desired to place a compliant element to actuate the packing in a sootblower hub, by conventional design practices, it would be necessary to provide for the ability to compress the spring from its free length as it is being installed within the hub. This requirement would dictate that the hub be sufficiently long to accommodate compressing the spring from its free length to a compressed condition at which a desired preload level is generated. Although such designs using relatively long free length springs could be incorporated into sootblower hubs, the added length of the hub necessary to initially compress the spring would constitute additional sootblower "dead space" which is of concern to boiler makers. Dead space in this context can be defined as the amount that the length of the sootblower extending from the boiler wall exceeds the distance that the lance tube is projected into the boiler.
In addition to concerns about increasing the length of the hub, live loaded spring biased packing would typically require a degree of operator skill and training in setting a desired preloaded force level. There is a constant desire to improve the reliability and repeatability of sootblower and facilitate their replacement and repair. Accordingly, the elimination of special procedures and training in packing adjustment is preferred.
In addition to the concerns expressed previously, there is a desire, when using a compliant element to load the packing material, to protect the compliant element from the hostile environment within the proximity to boiler and to shield the element from contamination and temperature extremes.
The hub assembly in accordance with the present invention provides the previously described desirable features. These features are provided by employing a novel packing gland system of the invention. One of the components is a tubular gland follower which acts on the packing through a bushing. Surrounding the gland follower is a collar which threads into the sootblower hub. Both the collar and gland follower have surfaces which engage the ends of a compression spring.
During assembly of the above mentioned components, the spring is installed and the follower and collar are forced together, compressing the spring to a level which provides the desired packing preload. Thereafter, a preload ring is installed onto the gland follower which abuts against the collar preventing these parts from becoming separated and maintaining the spring in a compressed state even before the packing gland is installed into the hub. This design enables the packing gland to be mounted into the hub, and once the elements are properly positioned with the gland follower engaging the packing bushing, the preload ring can be disengaged from the collar, allowing the spring preload to be transferred into the packing.
The hub design of this invention reduces the hub length which would otherwise be required for preloading a loading spring, and also provides a protected environment for the spring. Moreover, the system enables the packing gland to be preassembled with a desired preload level thus reducing the chance of incorrect usage or maladjustment in the field. This invention further provides improved packing performance, reduces maintenance, and improves cycle life. The features of this invention are further readily adaptable to existing sootblowers, providing retro-fit capability.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.